In a wireless communication system adopting distributed access (such as a Wireless Local Area Network (WLAN)), an access premise of its channel is that all stations (STAs), including common user-oriented stations and Access Points (APs), are viewed as equal STAs. An access manner is that each STA randomly generates backoff time in a contention window (CW) and contends for obtaining the channel by decreasing to zero. An advantage of the manner is that all the stations can obtain an access opportunity from the angle of a statistical probability, and there is no need of a dedicated device with powerful functions to implement central scheduling.
Specifically, for a quality of service (QoS) STA (QSTA), for example, all services are classified into 4 types: AC Video (AC_VI), AC Voice (AC_VO), AC Best Effort (AC_BE) and AC Background (AC_BK); and then, on the basis of this, different services are assigned different contention windows CW[AC], and delays before contention of different services are different. In this way, on a premise of ensuring that all have access opportunities, priorities of the video and voice are improved and priorities of the other two types of services are relatively reduced from the angle of statistics, that is, the video and the voice have the higher priorities and the best effect data and the background stream have the lower priorities.
In an Enhanced Distribution Coordinate Access (EDCA) mode, 4 ACs of each QoS STA are all similar to non-QoS STAs, they not only need to contend with ACs of other STAs or other QoS STAs for a right to use the channel, but also contend with ACs of the present STA for the right to use the channel. For a certain AC, various possible situations are inevitable to trigger a backoff process (that is, adjust the contention window, take a value randomly in the contention window, decrease progressively along with time according to the value, and resend its contention channel after decreasing to zero).
If the AC and another AC of the same STA obtain a transmission opportunity (TXOP) at the same time, it is referred that the AC and the another AC (generally an AC of a higher priority) have an internal collision.
When contention occurs, the higher priority AC obtains channel access, and initiates data transmission.
However, if the higher priority AC obtains the channel access when contention occurs, the lower priority AC triggers the backoff process. Frequently triggering the backoff process will greatly affect fairness of the lower priority AC, and limit a service of the lower priority AC.